1. Field of the Invention
The present invention generally relates to systems for inspecting curved surfaces and, more particularly, to systems that inspect curved surfaces such as the ones formed by solder joints that connect electronic components of the surface mount type to circuit boards.
2. State of the Art
For assemblies employing integrated circuit packages, circuit malfunctions are often traceable to faulty solder connections between a board and the leads of a circuit package mounted on the board. This is especially true for circuit packages of the surface-mount type, since those packages have leads which are usually finer and more closely spaced and, hence, more difficult to solder correctly than leads on circuit packages of the dual in-line pin (DIP) type. Circuit packages of the surface-mount type are designed, as their name implies, for electrical connection to the surface of a circuit board. The leads of surface-mounted packages can have various shapes including "J" and gull-wing like shapes.
For mounting circuit packages of the surface-mount type to a circuit board, solder paste is placed onto minute solder pads that are precisely located on the board to match the pattern, or "footprint", of properly aligned leads on each circuit package. After the desired number of circuit packages are placed on the board with the leads of the packages embedded in the solder paste on the pads, the packages are permanently soldered into place. To achieve satisfactory electrical connections, each of the solder joints must be adequately completed.
To assure that solder joints adequately connect surface-mount circuit packages to printed circuit boards, inspection is required. Although inspection can be done manually, such techniques are time-consuming and tedious. Accordingly, efforts have been made to automate the inspection task. However, automated inspection of solder joints is quite difficult because of the variability in appearance of acceptable solder joints, because of the highly specular nature of solder surfaces, and because of the three-dimensional nature of solder surfaces.
One suggested system for automated inspection of solder joints at the leads of surface-mounted circuit packages is set forth in U.S. Pat. No. 4,668,939. Specifically, that patent discloses an automated inspection system which inspects solder bumps on chip carriers by using video cameras, i.e., cameras that depict a scene in two dimensions. According to the patent, images provided by the video camera are processed to provide one-dimensional intensitY plots which can be analyzed to detect missing, bridged or excessive solder bumps.